The invention described herein may be manufactured, used, and licensed by the U.S. Government for governmental purposes without the payment of any royalties thereon.
1. Field of the Invention
This invention relates generally to uncooled infrared detectors and focal plane arrays, and more specifically to a method of fabricating a detector using rock salt as a removable substrate. The invention relates even more specifically to a method of fabricating a colossal magneto-resistive detector using a rock salt structure material.
2. Description of Related Art
Uncooled infrared thermal detectors have recently been developed into large-size focal plane arrays (hereinafterxe2x80x9cFPAxe2x80x9d). Use of a microbolometer is one successful method for infrared detection at room temperature. A microbolometer-type FPA typically employs vanadium oxide on silicon nitride with a micro-mechanically machined air bridge structure. The temperature coefficient of resistance for vanadium oxide is approximately 2%. The air bridge structure is built directly on a silicon readout integrated circuit (xe2x80x9cROICxe2x80x9d). Growth of detector materials directly on the ROIC restricts the material thin film growth temperature to less than 550xc2x0 C. as a result of the thermal budget limitation associated with the ROIC.
The use of colossal magneto-resistive xe2x80x9cCMRxe2x80x9d) materials for uncooled infrared detectors is described in Goyal et al., A., xe2x80x9cMaterial Characteristics of Perovskite Manganese Oxide Thin Films for Bolometric Applications,xe2x80x9d Applied Physics Letters, Vol. 71 (17) (Oct. 27, 1997), pp. 2535-2537. CMR materials demonstrate an exceptionally large change in resistance with temperature as they transition from a ferromagnetic to a non-ferromagnetic phase. The transition temperature can be adjusted through appropriate selection of materials and process conditions. The results have demonstrated the feasibility of growing CMR thin films on perovskite oxide material substrates such as LaAlO3 and SrTiO3 with a resultant temperature coefficient of resistance of greater than 7%. However, the temperature for growth of the CMR material, however, must be relatively high (i.e., greater than 700xc2x0 C.), which makes it difficult to grow directly on the ROIC.
CMR materials have a perovskite crystal structure with a square base. The lattice constantxe2x80x9caxe2x80x9d of the square base of a CMR material is approximately 3.8 to 3.9 xc3x85 depending on the material composition. As indicated above, CMR thin films have been successfully grown on perovskite oxide substrates such as LaAlO3 and SrTiO3, and exhibit a good crystal orientation and a high temperature coefficient of resistance. These perovskite oxide substrate materials are employed because of the correspondence of their crystal structure and lattice constant to those of CMR materials. For example, SrTiO3 has a cubic crystal structure with a lattice constant of 3.905 xc3x85, and LaAlO3 has a pseudo-cubic crystal structure with a lattice constant of 3.79 xc3x85. These properties facilitate the growth of a CMR material on LaAlO3 and SrTiO3 with a resultant high crystal orientation and quality. The detector material can be bonded to a ROIC, then the substrate is removed. A disadvantage associated with use of these materials, however, is that both LaAlO3 and SrTiO3 are very difficult to remove by etching once the detector array has been bonded to the ROIC.
Therefore, a general need exists to provide a method of fabricating an uncooled infrared detector which both satisfies the temperature coefficient of resistance and fabrication temperature constraints, and also provides a detector of the requisite film quality. An even more specific need exists to provide a CMR transferred thin film method in which the substrate can be easily removed.
It is an object of the present invention to provide a method of fabricating an uncooled infrared detector that produces a detector of the requisite film quality, satisfies the temperature coefficient of resistance, and easy to fabricate. Accordingly, in a first preferred embodiment, the present invention advantageously relates to a method of fabricating a CMR detector using a thin film transfer method with a rock salt structure material as a substrate. The method comprises (a) growing a protective layer comprising a lattice matched template material so as to envelop a rock salt structure material substrate; (b) depositing a colossal magneto-resistive layer on a deposition surface of the protective layer; (c) fabricating a detector array (d) bond the detector array to a ROIC; and (e) removing the rock salt structure material substrate from the bonded structure.
By using a rock salt structure material such as, for example, NaCl, LiF, NaF, KF, or KCl as the substrate, a high quality epitaxial CMR material with a high temperature coefficient of resistance can be fabricated. The substrate can be easily removed using water, and the excess rock salt structure material/water solution can then be removed by a method such as evaporation, triple-point, or freeze drying.
In a second preferred embodiment, the present invention relates to a method in which a perovskite oxide material such as, for example, LaAlO3 or SrTiO3 is employed as the substrate, and the rock salt structure material is employed as a buffer layer, template layer, and release layer. The method comprises (a) growing a rock salt structure material layer on a perovskite oxide material substrate; (b) growing a protective layer comprising a lattice matched template material on the rock salt structure material layer; (c) depositing a colossal magneto-resistive layer on the protective layer; (d) fabricating a detector array; (e) bonding the detector array to a ROIC; (f) removing the rock salt structure material with water or other solution; and (g) the substrates falls off and is removed.
By employing either of the aforementioned embodiments, the growth and processing temperatures can be higher than those associated with a conventional technique in which the CMR material is grown directly on top of the ROIC, thus yielding a CMR material of higher quality and higher crystal orientation, and higher temperature coefficient of resistance.
Advantages associated with the embodiments of the present method include not only the ability to produce a detector of the requisite film quality, but one which satisfies the temperature coefficient of resistance and fabrication temperature constraints. In addition, once the fabrication is complete, the substrate can be easily removed.